Use of selected inhibitors against the formation of solid organo-based incrustations from fluid hydrocarbon mixtures

ABSTRACT

The invention relates to the use of 
     (A) esters of phosphoric acid and/or partial salts thereof with alkoxylated aliphatic, cycloaliphatic and/or aromatic alcohols and/or 
     (B) fatty acid oligo-dialkanolamides as inhibitors against the formation of incrustations based on organic solids from liquid and/or gaseous hydrocarbon mixtures which are fluid and prone to form such incrustations. Preferred inhibitors having a surface-active character are capable of wetting, and especially of spreading on, solid work material surfaces also in the presence of the fluid hydrocarbon phase. They are particularly suitable as additives to crude oils and/or oil fractions susceptible of forming incrustations.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a proposal for an improved handling of fluidhydrocarbon mixtures having a tendency forming solid organic deposits,especially based on hydrocarbons, upon storage and/or when processed,which deposits then will remain as undesired incrustations in theinterior of pipelines, pumps, filters, containers and the like andrequire a considerable expenditure for cleaning. Particularly concernedby this problem are the areas of extracting, storing and/or conveyingcrude oils having high contents of paraffinic hydrocarbons and/or ofcorrespondingly handling oil fractions containing such hydrocarboncompounds. Nevertheless, comparable problems may also arise inextracting and processing natural gas; hereto cf., for example, F.Kerekes et al., “Maβnahmen gegen Paraffin-Ablagerungen in der Erdöl- undErdgasförderung”, Erdöl-Erdgas-Zeitschrift 99 (4/1983), 110-118 and theliterature quoted therein.

Functional disorders caused by a deposition of proportions of organicsolids—and more particularly paraffins and/or asphaltenes—contained inhydrocarbon mixtures that are fluid under normal conditions are variousin nature and, thus, become manifest in a great variety of ways. Oneimportant parameter in this respect is the temperature range of the pourpoint or of the setting point of the liquid hydrocarbon mixture. In thetemperature range exceeding this value, the fluidity would be basicallyensured, while the hydrocarbon mixture will solidify at temperaturesbelow said value. Crude oils and/or hydrocarbon fractions havingcomparably high pour and setting points may be improved with respect tothe flow property thereof also at reduced temperatures by the additionof so-called Pour Point Depressants (PPD's). The addition of PPD's, dueto an interaction with the amounts of solids crystallizing in the liquidphase, reduces the limiting temperature below which the fluidity of thehydrocarbon mixture is lost.

However, independently of the problem area referred to herein, thereexists a complex second problem area for the practical handling of suchliquid hydrocarbon mixtures, but also of gaseous hydrocarbon mixtures.This is related to the incrustations which in practice give rise to avariety of disorders and have been formed by solid hydrocarbon compoundson the inner walls of, for example, pipelines, pumps, on filters or instorage tanks, which incrustations occur even if the fluidity of thehydrocarbon mixture itself is retained. Thus, for example, the interiorof pipelines for the transportation of crude oil must be regularlysubjected to cleaning procedures which are expensive in time and costs,because in the regular operation of conveying oil incrustations of solidhydrocarbon compounds are increasingly formed on the inner wall of thepipelines. The working order of pumps and filters is threatened by suchformation of incrustations during the continuous operation, ifincrustations of this type are uninhibitedly permitted to build up.Particular threats in thus directions are caused also by that thenormally flowing liquid hydrocarbon-based material is intermittentlyexposed to inevitable standstill periods.

2. Discussion of Related Art

Hitherto the efforts made in the art for finding solutions to theseproblems have had only a restricted practical success. What has beendesired is, by adding efficient inhibitors to prevent, or at least toreduce, this undesirable tendency towards incrustation and moreparticularly the strong adhesion of the, e.g., paraffinic incrustationsto the solid surfaces being in contact with the liquid and/or gasphases. Here, the addition of the known PPD's lowering the settingand/or pour point(s) of the kind discussed above does not provideremedy. Really useful inhibitors for the goal according to the inventionso far have not been proposed in connection with liquid hydrocarbonmixtures. The literature reference quoted above fromErdöl-Erdgas-Zeitschrift 99 deals with paraffin deposits from naturalgas condensates, among others. Here, laboratory experiments werereported for developing paraffin inhibitors based on wetting agentsand/or dispersants. Such wetting agents acting through a water phaseform a surface film on the walls which largely counteracts a depositionof paraffins and/or infiltrates and mobilizes films already present,respectively. More specifically, for practical use there is proposed acombination of a nonylphenol ethoxylate and a dodecylbenzyl sulfonate.

The teaching according to the invention is based on the recognition thattwo substance classes preferably having a surfactant character asdescribed in detail hereinbelow are capable of providing remedy to theproblems concerned here in a hitherto unknown manner. These auxiliaryagents of the invention, hereinbelow designated as the active substances(A) and (B), are obviously distinguished by that they are capable ofstriking and spreading on the solid work materialsurfaces—conventionally metal surfaces—and here counteract theattachment and growth of solid hydrocarbon deposits and, hence, theundesirable incrustation, supposedly so due to the chemical naturethereof.

DESCRIPTION OF THE INVENTION

Thus, the invention relates to the use of

(A) esters of phosphoric acid and/or partial salts thereof withalkoxylated aliphatic, cycloaliphatic and/or aromatic alcohols and/or

(B) fatty acid oligo-dialkanolamides

as inhibitors against the formation of solid hydrocarbon incrustationsfrom hydrocarbon mixtures which are fluid and prone to form suchincrustations. The term “aromatic alcohols” as used in the context ofthe component (A) includes phenols and corresponding compounds havingthe OH group(s) bonding to the aromatic moiety.

It is preferred to employ inhibitors according,to (A) and/or (B) whichexhibit surfactant character and, in addition, are capable of-wet,tigsolid work material surfaces and especially of spreading on solid workmaterial surfaces also in the presence of the fluid hydrocarbon phase,even if this is comprised by liquid hydrocarbon mixtures. The inhibitorsof the invention according to (A) and/or (B) are especially suitable forthe protection of metallic work material surfaces which are in contactwith the fluid hydrocarbon phase tending to form incrustations.

Re the definition of the inhibitors (A) according to the invention:

The active substances of this class of inhibitors used according to theinvention are esters of phosphoric acid with selected alcohols. Includedin the invention is the use of the phosphoric acid triesters. Inpreferred embodiments there are employed partial esters or mixtures ofpartial esters of phosphoric acid and the selected alcohols still to bedescribed herein-below. In the case that such partial esters are used,it may be expedient to utilize the non-esterified acid groups in theform of salts thereof, and especially in the form of alkali metal salts,ammonium salts, alkanolamine salts and/or amine salts. As the inhibitorcomponent (A) there may be particularly suitable mixtures of mono-and/or diesters of phosphoric acids and/or their respective salts.

Within the scope of this inhibitor component (A) there are employed, asthe ester-forming alcohol components, alkoxylated aliphatic,cycloaliphatic and/or aromatic alcohols which preferably contain atleast 3, especially at least 4 to 6, carbon atoms in the aliphatic,cycloaliphatic and/or aromatic alcohol moiety. A particularly suitableclass of ester-forming alcohols is constituted by alkoxylated phenol-derivatives, among which alkoxylated alkylphenols may be of particularimportance as phosphoric acid ester-forming alcohol components.Alkoxylated fatty alcohols constitute a further preferred class ofcompounds for composing the inhibitors (A).

As the alkoxylated products of said alcohols within the scope of theinvention there are especially utilized the ethoxylated and/orpropoxylated derivatives of monofunctional alcohols or phenols,respectively. The degree of alkoxylation may be, for example, within therange of from 1 to 20, and especially within the range of from about 3to 15, and is in detail in a per se known manner also dependent on thesize of the respective hydrocarbyl moiety in the monofunctional alcohol.Thus, e.g., ethoxylated nonylphenols having an average of 5 to 10ethylene oxide (EO) units per 1 mole of nonylphenol have proven to bevery suitable as balanced and efficient alkoxylated alcohols for theformation of the inhibitor component (A) within the scope of theinvention. One preferred component (A) for the teaching according to theinvention comprises mixtures of the sodium salts of mono- and diestersof phosphoric acid with such nonylphenol/5-10 EO adducts. EO adducts offatty alcohols of the range of C₈₋₂₀, and especially C₁₀₋₁₈, comprisingfrom 1 to 20 EO units, and preferably about from 3 to 15 EO units, are afurther important class for the formation of the inhibitors under (A).

The inhibitors used according to the invention of the active substanceclass (B) are fatty acid oligo-dialkanolamides. Within this meaning, theterm fatty acid includes monocarboxylic acids containing at least 6 to 8carbon atoms, while acids containing at least 10 carbon atoms and aboveall monocarboxylic acids of the range containing 12 and more carbonatoms are especially preferred. The carboxylic acid, in consideration ofthe explanations further provided hereinbelow, may be saturated and/orunsaturated. Olefinically mono- and/or polyunsaturated carboxylic acids,particularly from the range of C₁₂₋₂₄, are particularly suitablerepresentatives.

In the inhibitor component (B) according to the invention the carboxylicacids are present as oligo-dialkanolamides, i.e. as carboxylic acidamides from the reaction with a limited excess of dialkanolamines.Preferred as the dialkanolamines are compounds having comparably lowhydroxyalkyl moieties, especially with corresponding moieties having upto 5, and preferably up to 3, carbon atoms. Diethanolamine may beespecially suitable for the preparation of the inhibitor component (B).

Suitable to serve the purpose according to the invention are reactionproducts of fatty acids prepared by using a slight excess of thedialkanolamine over the amount stoichiometrically required for theformation of the fatty acid amide. Especially suitable are inhibitorcomponents (B), in the preparation of which the fatty acid-basedreaction component has been reacted with an at least about 0.5 molarexcess of the dialkanolamine. Suitable may be reaction products of fattyacids and dialkanolamines which comprise the dialkanolamine in an about0.5 to 3.0 molar excess, relative to the amount of carboxylic acid. Ifthe reaction of the reactants forming the inhibitor component (B) iscarried out at sufficiently high temperatures—for example attemperatures in excess of 175° C. to 180° C.—then the formation of thecarboxylic acid amide proceeds, on the one hand, while, on the otherhand, an extension of the chain of the amide-forming reactant iseffected due to a removal of water and an ether formation between thefree hydroxyl groups of the dialkanolamines, as is shown hereinbelow foran idealized reaction between one mole of a carboxylic acid RCOOH andtwo moles of diethanolamine:

At reaction temperatures in the range of about 190° C. a fatty acidamide having the idealized formula

is obtained. Possible is also a linkage with the formation of a tertiaryN atom in the chain-extended group.

For the effect provided by the inhibitor component(s) (A) and/or (B)according to the invention it can be important that they have somefluidity. In this respect, more particularly, in the selection of thecarboxylic acid amide compounds, olefinically mono- and/orpolyunsaturated carboxylic acids may be of importance. The respectivefatty acids of natural origin, and among these especially olefinicallyunsaturated fatty acid mixtures are particularly suitable for thepreparation of the inhibitor-components (B). Known as one significantexample for this are tall oil fatty acids a by far predominantproportion of which consists of a mixture of olefinicallymonounsaturated, di-unsaturated and tri-unsaturated C₁₈-monocarboxylicacids. Other suitable natural feed materials for unsaturated fatty acidsare vegetable ester oils comprising olefinically monounsaturated and, asthe case may be, polyunsaturated carboxylic acids, especially those ofthe range of C₁₆₋₂₄. For example, palm oil, peanut oil, castor oilsoybean oil, fish oil and especially rapeseed oil may be mentioned here.

Each of the inhibitor components (A) and (B) as defined according to theinvention may be put into use alone by itself in the hydrocarbon mixturesusceptible of forming incrustations. However, it is preferred to usemixtures of (A) and (B), where the broad range of ratios of amounts ofA/B of from about 5/95 to 95/5 is contemplated. Preferred mixing ratiosare within the range of from about 25/75 to 75/25 and more particularlythose within the range of from about 40/60 to 60/40. The ratios ofamounts and/or ranges, respectively, are based on the ratio by weight ofthe components (A) and (B). Particularly equal inhibitor mixtures underthe teaching of the invention contain approximately equal amounts byweight of (A) and (B).

A number of technical options exists for rendering the wall areasintended to be protected inert to the undesired build-up ofincrustations. One embodiment comprises applying the inhibitors orinhibitor mixtures in a suitable auxiliary liquid onto the wall areas tobe protected and allowing them to be adsorbed thereon. For example,solutions of the inhibitors or inhibitor mixtures, respectively, insolvents can be applied onto the wall areas to be protected, for exampleby spraying, before the respective working device will be contacted withthe hydrocarbon mixture susceptible of forming incrustations.

Suitable as auxiliary liquids, more particularly, are appropriateaqueous and/or organic systems. The inhibitors used according to theinvention, in the preferred embodiment, possess some dissolving powerand/or self-emulsifying power in an aqueous phase, due to theirsurfactant character, and in this form may be conveyed to the wall areasto be protected or to other solid areas. However, solutions and/oremulsions of the inhibitors in organic or organic-aqueous liquid systemsare also suitable for use in such a pretreatment, due to the highaffinity to solid surfaces, more particularly those made of metal. Ifrequired, the homogeneity of the liquid phase for rendering the wallareas intended to be protected inert to the undesired build-up ofincrustations can be ensured by using auxiliary emulsifiers in a per seknown manner.

Nevertheless, the formation of the protective layers according to theinvention against the undesirable formation of incrustations is notrestricted to such pre-treatment of the regions to be protected. It ismuch easier, as a rule, to add comparably low amounts of the inhibitorsto the hydrocarbon mixtures which, in the absence of the additivesaccording to the invention, cause the undesired formation of theincrustations to take place. Thus, it may be appropriate to add thesurfactant inhibitors to liquid hydrocarbon mixtures in amounts of aboutfrom 5 to 1,000 ppm, preferably in amounts of about from 10 to 500 ppmand especially in amounts of about from 20 to 200 ppm. It has been shownthat even in such high application dilutions—which in practical use mayrange from about 30 to 100 ppm—the surface-active inhibitors areadsorbed on the wall areas made, for example, of metal, and hereefficiently attain the object according to the invention. Moreover, ithas been shown that an efficient protection can also be achieved, ifthis addition of the inhibitors according to the invention is effectedjust batchwise, for example as an additive to a crude oil stream passedthrough a pipeline. The interior wall areas remain inert to anundesirable paraffin deposition over a considerable period of time, evenif in this subsequent period the crude oil stream does not contain anyinhibitor components. The embodiments described here may also becombined.

In an important embodiment of the invention, the inhibitor components(A) and (B) as defined are used in combination with conventional settingpoint depressants, with respect to the nature of which reference may bemade to pertinent prior art. Commercial setting point depressants areknown to be, for example, branched polymer compounds having some longerresidues, for example acrylate polymers comprising moieties oflong-chain fatty alcohols—especially those of the range C₂₀₋₂₄—in anester bond to the polymer chain. Further examples of known flowimprovers or setting point depressants of the kind referred to here areto be assigned to the classes of the polyethylenes and theethylene-vinyl acetate copolymers add/or ethylene-vinyl propionatecopolymers. Even if the detailed mechanism of action of these productshas not been fully elucidated, they have in common that first themodifying auxiliary agents will have to be heated together with thecrude oil at temperatures much above the setting point. Upon subsequentcooling, interactions occur between the dissolved inhibitors of the PPDtype and the precipitating organic solids. Normally, the crystal growthwill he hindered, so that small particles are formed which do not permita formation of networks. As for details, reference is made to thepublication as initially quoted in Erdöl-Erdöl-Erdgas-Zeitschrift 99,113-114.

The inhibition of the incrustation within the scope of the teaching ofthe invention obviously is governed by different principles. Adsorptiononto the wall surfaces to be protected from an incrustation build-up andspreading on said wall surfaces of the inhibitors (A) and/or (B)according to the invention does not require the combination of flowcomponents to be heated, for example of the crude oil stream to beconveyed and inhibitors or inhibitor combination, respectively.

However, it was found that the concomitant use of the inhibitors (A) and(B) according to the invention may also exert an influence on themanifestation, for example, of the paraffinic solid crystals beingformed in the oil stream. Crystal formations like those formed below theso-called cloud point may also be affected by the concomitant use of theinhibitors (A) and (B) according to the invention. Investigations haveshown that a characteristic phenomenon may be represented by a reductionin size of the crystallites and, thus, the solids particles being insolid communication with each other. This phenomenon may give a positiveeffect onto the flow behavior of the oil stream containing solidprecipitates, even if this would not necessarily involve a decrease inthe setting point.

EXAMPLES Experimental Setup

A reservoir having a capacity of 5 liters for holding the particular oilto be examined is provided with a stirrer, a thermometer for measuringthe oil temperature, an oil withdrawal pipe and an oil feed pipe. Saidfeed and withdrawal pipes are part of an oil circulation system operatedby means of a pump circulating the oil with a controllable flowvelocity.

In this circulation piping for the circulated oil stream there isprovided one pipe piece made of iron filled with a dense packing ofRaschig rings. Said iron pipe portion is located in a cooling bath thetemperature of which is thermostated during the respective period of theoperation. Also the temperature of the oil stock in the reservoir isadjustable and/or maintainable to a pre-determined value by thermostaticcontrol during the circulating operation.

The liquid pressure of the oil stream withdrawn from the reservoir ismeasured at the point directly before said oil stream enters the Raschigring-filled iron pipe portion and is displayed by an appropriatepressure gauge.

Conditions of Process Operation

The temperature of the oil contained in the reservoir is adjusted to apredetermined value which is by about 20° C. to 30° C. higher than thesetting point of each oil sample being investigated and thermostated atthat temperature.

The temperature of the cooling bath in which the Raschig ring-fillediron pipe portion of the circulated stream is immersed is adjusted to atemperature within a range up to about 4° C. higher than the settingpoint of the employed oil sample and is maintained at that preset value±1° C.

The pump delivery rate in the circulated oil stream is set to apredetermined value and kept constant at said value for the duration ofthe experiment. In the tests described hereinbelow, the pump deliveryrate of the liquid stream is 15 ml/min.

One test cycle normally takes 8 hours. Due to the formation ofincrustations in the Raschig ring-filled and cooled iron pipe portionthere is observed an increase in the pressure of the oil stream directlyin front of the entrance into the cooled pipe portion, said pressureincrease being caused by the degree of incrustations formed and, hence,the reduction in the cross-section of the flow path. Each test cycle isterminated before the end of the 8 hour period, once the pressureincrease amounts to 50% of the initial oil pressure in the beginning ofthe test cycle.

Experimental Results

A paraffin-containing crude oil having a setting point determinedaccording to ASTM D 97 B (maximum setting point) of 18±1° C. is adjustedto the regular temperature of 40° C. in the reservoir. The temperatureof the cooling bath for the Raschig ring-filled iron pipe portion ismaintained at 22° C.

The pressure measured immediately after starting and adjusting the oilcirculating process in the withdrawal pipe directly in front of theentrance into the cooled pipe portion is about 2 bar.

A first test is carried out in the absence of inhibitors according tothe invention. After the oil was circulated under the conditions setforth above for a period of 4.5 hours, the pressure at the point ofmeasurement in the oil stream had increased by 50%. Then, the experimentwas stopped.

In the subsequent tests, an inhibitor mixture of the components (A) and(B) according to the invention as defined hereinbelow is employed:

Inhibitor Component (A)

A mixture of phosphoric acid mono- and diesters is produced by reacting2.8 moles of nonylphenol +6.5 EO with 1 mole of P₂O₅. The acidic estermixture is converted into the corresponding salts with thestoichiometric amount of 50% aqueous sodium hydroxide solution. A minoramount of nonylphenol +6.5 EO is added to the substance mixture as anauxiliary emulsifier (about 23% by weight of auxiliary emulsifier,relative to the total mixture).

The inhibitor is present in the form of a high viscosity liquid phasewhich is emulsifiable/soluble in water and liquid organic solvents.

Inhibitor Component (B)

Employed is a tall oil fatty acid oligo-diethanolamide from 1 mole oftall oil fatty acid (about 2% by weight of residual resin) and 2 molesof diethanolamine. The product obtained at a reaction temperature of190° C. has an amine value of 123 and an acid value of 4.4. It is areaction product which is fluid under normal conditions.

In the following experiments, the inhibitors (A) and (B) are employed asan admixture comprising equal amounts by weight of either.

Example 1

The inhibitor mixture according to the invention is, added in an amountof 30 ppm to the crude oil and uniformly distributed therein, before theoil circulation stream is started. Then, circulating oil flow ismaintained under the process conditions as reported over a period of 8hours. After the end of the test period, the pressure increase in theoil stream at the entrance into the Raschig ring-filled pipe portion wasabout 5%.

Example 2

The procedure of Example 1 is repeated, except that the inhibitormixture according to the invention is added in an amount of 250 ppm tothe crude oil.

Upon termination of the test after 8 hours, any noticeable pressureincrease in the circulated oil stream is not determinable.

Example 3

Prior to the begin of the test, the Raschig ring-filled pipe portion ofthe circulation system is immersed over night in an aqueous 2% by weightemulsion of the inhibitor mixture according to the invention. Theaqueous phase is allowed to drain off from the treated iron pipe piecewhich is then inserted into the designed circulating system. Then thecrude oil capable of undergoing paraffin precipitation is charged intothe reservoir without adding any additional inhibitors according to (A)and/or (B), and the oil circulation process is initiated under thedescribed conditions. After a process operation over 8 hours, anysubstantial pressure increase in the circulated oil stream in front ofthe entrance into the cooled pipe portion is not determinable.

What is claimed is:
 1. The process of inhibiting the formation of solidhydrocarbon incrustations from hydrocarbon mixtures which are fluid andprone to form such incrustations, comprising contacting said hydrocarbonmixtures with an inhibitor mixture comprising (a) esters of phosphoricacid or salts thereof and alkoxylated aliphatic, cycloaliphatic oraromatic alcohols, and (b) fatty acid oligo-dialkanolamides.
 2. Aprocess as in claim 1 wherein said inhibitor mixture is surface-activeand capable of wetting or spreading on solid work material surfaces inthe presence of said hydrocarbon mixtures.
 3. A process as in claim 1wherein said inhibitor mixture is applied in an aqueous or organicliquid to surface areas to be protected from incrustation.
 4. A processas in claim 1 wherein said hydrocarbon mixtures are contacted with fromabout 5 to about 1,000 ppm of said inhibitor mixture.
 5. A process as inclaim 1 wherein said inhibitor mixture is present in a weight ratio of(a) to (b) of from about 5:95 to 95:5.
 6. A process as in claim 1wherein said esters of phosphoric acid or salts thereof comprisealkoxylated phenol compounds.
 7. A process as in claim 6 wherein saidesters of phosphoric acid or salts thereof comprise a mixture of thesalts of mono- and diesters of phosphoric acid with nonyl phenolcontaining 5 to 10 moles of ethylene oxide.
 8. A process as in claim 1wherein said fatty acid oligo-dialkanolamides comprise the reactionproducts of saturated or unsaturated C₁₂-C₂₄ monocarboxylic acids and amolar excess of dialkanolamines.
 9. A process as in claim 1 wherein saidfatty acid oligo-dialkanolamides comprise the reaction products ofunsaturated C₁₆-C₂₀ carboxylic acids and a 0.5 to 3.0 molar excess ofdiethanolamine.
 10. A process as in claim 1 wherein said fatty acidoligo-dialkanolamides comprise the reaction products of tall oil fattyacids and diethanolamine in a molar ratio of from 1:1.5 to 3 whereinwater is removed, and said reaction products have a large degree ofoligomerization of said diethanolamine.
 11. A process as in claim 1wherein said inhibitor mixture is employed for the protection ofpipelines, pumps, filters, tanks and metal surfaces against theformation of incrustations thereon.
 12. The process of inhibiting theformation of solid hydrocarbon incrustations from hydrocarbon mixtureswhich are fluid and prone to form such incrustations, comprisingcontacting said hydrocarbon mixtures with 10 to 500 ppm of an inhibitormixture comprising (a) esters of phosphoric acid or salts thereof andalkoxylated aliphatic, cycloaliphatic or aromatic alcohols, and (b)fatty acid oligo-dialkanolamides.
 13. A process as in claim 12 whereinsaid inhibitor mixture is surface-active and capable of wetting orspreading on solid work material surfaces in the presence of saidhydrocarbon mixtures.
 14. A process as in claim 12 wherein saidinhibitor mixture is applied in an aqueous or organic liquid to surfaceareas to be protected from incrustation.
 15. A process as in claim 12wherein said inhibitor mixture is present in a weight ratio of (a) to(b) of from about 5:95 to 95:5.
 16. A process as in claim 12 whereinsaid esters of phosphoric acid or salts thereof comprise alkoxylatedphenol compounds.
 17. A process as in claim 16 wherein said esters ofphosphoric acid or salts thereof comprise a mixture of the salts ofmono- and diesters of phosphoric acid with nonyl phenol containing 5 to10 moles of ethylene oxide.
 18. A process as in claim 12 wherein saidfatty acid oligo-dialkanolamides comprise the reaction products ofsaturated or unsaturated C₁₂-C₂₄ monocarboxylic acids and a molar excessof dialkanolamines.
 19. A process as in claim 12 wherein said fatty acidoligo-dialkanolamides comprise the reaction products of unsaturatedC₁₆-C₂₀ carboxylic acids and a 0.5 to 3.0 molar excess ofdiethanolamine.
 20. A process as in claim 12 wherein said fatty acidoligo-dialkanolamides comprise the reaction products of tall oil fattyacids and diethanolamine in a molar ratio of from 1:1.5 to 3 whereinwater is removed, and said reaction products have a large degree ofoligomerization of said diethanolamine.
 21. A process as in claim 12wherein said inhibitor mixture contains a setting point depressant.